(1) Field of the Invention
The present invention relates to a ball valve which comprises a valve box having inlet and outlet openings, and a spherical valve body rotatably arranged in the valve box.
(2) Description of the Prior Art
Conventionally, such a ball valve mounts an O-ring on each of the valve seat surfaces which are formed with an end portion of each of the inlet and outlet openings which are positioned inside the valve box, and the spherical valve body is forcibly contacted with the O-rings so as to keep a closing state.
However, an ordinary O-ring is inferior in heat resistance, pressure tightness and chemical resistance and therefore such a ball valve is given a limitation with respect to places and conditions of fluids flowing therethrough.
It was proposed to directly contact the valve seat with the spherical valve body in the manner of metal to metal contact. Such a construction caused another problem for the spherical valve body tended to seize up in contact with the valve seat at the time of opening and closing of the flow openings with rotating operation of the spherical valve body.
In view of the fact, it might be proposed to carry out the contact between the spherical valve body and the valve seat lightly so as to prevent seizure of the spherical valve body. However, such a manner could not carry out closing and opening of the ball valve precisely and as the result caused leak of fluid.